Electrostatic pinning of polymeric film

ABSTRACT

An apparatus for electrostatic edge pinning of thermoplastic film to a quench surface comprising a pinning wire positioned substantially parallel to the quenching surface and over each edge of the polymeric film.

United States Patent Gillyns et al.

[ 51 Sept. 11, 1973 ELECTROSTATIC PINNING OF [56] References CitedPOLYMERIC F UNITED STATES PATENTS Inventors: Emile Gillyns, Sandweiler;Norbert 3,571,853 3/1971 Demon 425/174 Thill, Itzig, both of Luxembourg3,174,748 3/1965 Roberts et al. 271/51 X 3,039,388 6/1962 Brandt et al226/94 x Assignee:

Filed:

E. I. du Pont de Nemours and Company, Wilmington, Del.

Dec. 2, 1971 Appl. No.2 204,072

Primary Examine r-H. A. Kilby, Jr. Attorney Donald W. Huntley [57]ABSTRACT An apparatus for electrostatic edge pinning of thermo- U.S.c|..... l n h ff Int. (:1 B2911 5/24 p lg. g i f'ff 'fl a fil Field ofSearch 425/174, 174.8; S y par 6 quenc mg rf h l 264,22 su ace and overeac edge of the po ymerlc film 3 Claims, 4 Drawing Figures ELECTROSTATICPINNING OF POLYMERIC FILM BACKGROUND OF THE INVENTION The preparation ofmany types of polymeric film involves extruding a molten sheet ofpolymer from a slit die onto a cooled quenching surface.

In the past, electrostatic pinning means have been used to force thefreshly extruded film into more intimate contact with the quenchingsurface. One apparatus commonly used is a small diameter wire extendingover the full width of a freshly extruded web immediately above thetouchdown point, a high voltage being passed through the wire to forcethe web into contact with the grounded quench drum. Unfortunately, withincreased casting speeds, an effect known as pinner bubbles develops inwhich small quantities of air are occluded between the extruded web andthe surface of the drum, resulting in optical defects in the finishedfilm product. Accordingly, at exceptionally high production rates, pointpinning probes are used for electrostatically pinning only the edges ofthe freshly extruded film, thereby preventing undesirable neck-in ofthefilm as it cools. Edge probes of this type commonly used in the artinclude those described in Owens & Vieth, U.S. Pat. No. 3,223,757.

The use of such point probes, however, has heretofore resulted in asubstantial indentation in the film at the point at which the electrodeis directed. This indentation renders the margin of the film unsuitablefor commercial use, due to the irregularity of the thickness at thispoint. In addition, this thinner portion of the film resulting from theuse of point probes can cause breakage in the transverse orientationprocess because of the weakness of the film at this point.

SUMMARY OF THE INVENTION The instant invention provides an improvedapparatus and process for electrostatic edge pinning that alleviates thedifficulties previously encountered with point probes.

Specifically, the instant invention provides an apparatus for pinningthe edges of a molten, thermoplastic, polymeric film extruded from a dieonto an electrically grounded quenching surface, which apparatuscomprises at least one uninsulated wire electrode, connected to a highvoltage source and positioned above the approximate touchdown point ofthe film onto the quenching surface and substantially parallel to thequenching surface, the uninsulated electrode being above only thoseportions of the film adjacent each lateral edge of the film, andpreferably within the percent of the width of the film adjacent eachedge.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of anelectrode support apparatus that can be used in the instant invention.

FIG. 2 is a perspective view of an extrusion apparatus using theelectrode arrangement of the instant invention.

FIGS. 3 and 4 are representative cross-sectional illustrations,respectively, of the edges of films having been pinned to a quenchingsurface using a conventional point probe and an electrode arrangement ofthe instant invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is based onthe discovery that one or more lengths of pinning wire, parallel to theweb, can be used for edge pinning a molten extruded web, in applicationswhere the concentrated force of an electrostatic point probe waspreviously used. At the same time, the use of pinning wires foredgecontrol results in less damage to the film product during pinning,facilities subsequent transverse directorientation and provides ease ofadjustment of the pinning force during operation.

The pinning wire used in the instant invention can be of the typegenerally used in the art, as described, for example, in Owens et al.,US. Pat. No. 3,223,757, hereby incorporated by reference. A fundamentalrequirement of the invention is that uninsulated sections of the wire,that is, those sections not surrounded or shielded from the groundeddrum by dielectric, be positioned over each edge of the extruded film toimpart the required pinning force. Accordingly, the supports for thepinning wire can vary widely, including apparatus in which pieces ofwire are separately supported over each edge of the web. Preferably,however, for maximum flexibility in the operation of the presentapparatus, a single wire is supported .over the full width of theextruded web, as with full width pinning, except that the centralportions of the wire are insulated by a suitable means to prevent thedeposition of ions at all points except the edges of the film.

Apparatus that can be used for the support of the wire electrode inaccordance with the instant invention includes supports of the typeillustrated in FIG. 1 and described in detail in Denton et al., US. Pat.No. 3,571,853, hereby incorporated by reference. In apparatus using twoseparate wires positioned at the outer edges of the extruded film, twolateral supports of the type illustrated in FIG. 1 as element 10 can beused to maintain the pinning wire above the edges of the extruded weband parallel to the surface. However, to permit maximum flexibility inthe adjustment of the length of uninsulated pinning wire over the edgeportions of the film, it is preferred that two such lateral supports beused to bear a wire across the full width of the extruded web, incombination with insulating members on the middle portion of the wire.Such an arrangement is illustrated in schematic cross-section in FIG. 1in which electrode support element 10 is provided with movable sleeve 11for adjustment of the uninsulated portion 12 of electrode wire 13. Thecentral portion of the electrode wire is covered with fixed insulation17 as well as movable insulating sleeve 14. The movable sleeves 11 and14 have plugs 15 and 16 with apertures formed therein through which theelectrode can pass, and the insulating sleeves fit snugly overinsulation 17 of the electrode as well as the lateral support. Thelength of the uninsulated portion of the electrode can be adjusted bythe positioning of sleeves I1 and 14, even when the pinning apparatus isin operation, using a dielectric stick or wand.

In a pinning operation using the apparatus of FIG. 1, a correspondingapparatus 10 supports the other end of the wire with a seconduninsulated portion positioned over the opposite edge of the extrudedweb. Such an arrangement is illustrated in perspective view in FIG. 2,in which molten web 19 is extruded from hopper 20 onto metal quench roll21. Mounted on frame 22 are lateral supports and 10' which support wire13 in a taut condition as it spans roll 21, parallel to the rollsurface, close to the normal touchdown point of web 19 as it is cast onthe rotating drum. The location of uninsulated electrode sections 12 and12, at the edges of the extruded web, are such that the outer edge ofthe uninsulated portion is just inside the edge of the web. Theinsulating sleeves 11 and 11 should be adjusted so that the exposed wiredoes not have a free electrical path to the roll, to prevent sparkingbetween the wire electrode, charged from voltage source 23, and thegrounded quench drum.

The insulating material used to form the dielectric sleeves in thepreferred apparatus of the instant invention can include any dielectricthat can be formed into a substantially rigid cylinder, such as hardrubbers and resins, e.g., fluorocarbon resins, urea formaldehyde resins,filled phenol formaldehyde resins, nylons and cast epoxy resins. It hasbeen found that Teflon fluorocarbon resins are particularly well suitedfor this application, being stable at temperatures encountered in thevicinity of the casting hopper, and having excellent insulatingproperties. The insulating coating.l7 for the wire electrode can beprepared from the same materials as the dielectric sleeves as well asother insulating materials such as polyimides, polyketones, andpolyolefins.

In operation of the present apparatus, the uninsulated portion of thewire electrode will necessarily vary with the thickness of the film, thewidth of the extruded web, the speed of the extrusion operation and theparticular polymer involved. In general, to obtain the henefit of edgestabilization associated with edge pinning, the uninsulated portions ofthe wire electrode at each edge should comprise at least about 0.5centimeters in length each. However, each uninsulated portion of theelectrode should in no case extend beyond about 5 percent of the totalwidth of the extruded film, and preferably less than 3 percent, toprevent the surface, defects normally associated with full wire pinningat exceptionally fast extrusion rates.

In the operation of the apparatus, maximum edge stabilization withminimum distortion of the web is obtained by centering the pinning forceover the bead or thickened portion at the edge-of the extruded web.

The use of the instant apparatus effects a remarable improvement in thecustomary weakening effect on the film edge that occurs whenconventional point probe pinning apparatus is used. This effect isillustrated in FIGS. 3 and 4, which are cross sectional illustrations ofthe edges of extruded webs having been pinned using point probes and theapparatus of the instant invention, respectively. In FIG. 3, web 19,having been extruded onto quench surface 21, is pinned to the quenchingsurface using point probe 30. A marked depression 31 develops in edgebead 32, causing a weakening of the web and contributing to waste byadditional shoulder 33 thrown up by the sharp pinning force, which mustbe slit off the useful web and discarded. By contrast, in FIG. 4, thepinningforce generated from uninsulated wire electrode 12 forms a gentlecurvature 41 away from the edge of the web.

By use of the apparatus of the instant invention, it has been found thatthe amount of waste due to edge trim from the web can be reduced by 25percent or more over the amount encountered in conventional point probepinning. In addition, the present apparatus is readily adjusted duringthe pinning process and of relatively simple construction. Further, theapparatus of the instant invention results in increased strength at theedge of the web, thereby reducing breakage during transverseorientation, in which the edges of the film are gripped by thestretching apparatus. Still another advantage of the invention is theability to readily adjust the location of the edge pinning force, forexample, to accommodate changes in the width of the extruded web.

If the procedures of the following example of the invention are carriedout, the indicated results will be obtained.

EXAMPLE A polyethyleneterephthalate film is melt extruded onto aquenching drum at a thickness of about 10 mils and a width of about 110centimeters. An apparatus of the type illustrated in FIG. 2 ispositioned above the touchdown point of the web onto the drum. Thepinning wire has a diameter of about 0.20 millimeter, with a voltageapplied thereto of about 8 kilovolts. Sleeves ll, 11', 14 and 14',fabricated of Teflon fluorocarbon resin, are adjusted to provide alength of uninsulated wire about 1.5 centimeters long, beginning overeach lateral edge of the web and extending inward. The pinning apparatussatisfactorily stabilizes the edge of the web.

The film is biaxially oriented by stretching 3.3 times the originaldimension of the film, in each of the machine and transverse directions,to give a final product having a thickness of about 1 mil. The edgeportions of the film are cut off. The weight percent of commerciallyacceptable film, after removal of edge trim, is about from 2-5 percentgreater, based on weight of extruded polymer, than is usually obtainedafter trimming in operations using pointprobes.

We claim:

1. In an apparatus for pinning a molten thermoplastic polymeric filmextruded from a die onto an electrically grounded quenching surface,which apparatus comprises atleast one single-strand wire electrodeextending across at least the full width of the extruded film, connectedto a high voltage source and postioned substantially parallel to thequenching surface above the approximate touchdown points of the film,the improvement which consists in at least the centermost percent of thewire electrode between the lateral edge portions of said film beinginsulated by a dielectric sheath and uninsulated portions being inproximity to thelateral edges of the molten film.

. 2. An apparatus of claim 1 wherein each of the lateral portions of thefilm in proximity to the uninsulated electrode comprises less than 5percent of the total width of the film.

3. An apparatus of claim 1 wherein each of the lateral portions of thefilm in proximity to the uninsulated electrode comprises less than 3percent of the total width of the film.

1. In an apparatus for pinning a molten thermoplastic polymeric filmextruded from a die onto an electrically grounded quenching surface,which apparatus comprises at least one single-strand wire electrodeextending across at least the full width of the extruded film, connectedto a high voltage source and postioned substantially parallel to thequenching surface above the approximate touchdown points of the film,the improvement which consists in at least the centermost 90 percent ofthe wire electrode between the lateral edge portions of said film beinginsulated by a dielectric sheath and uninsulated portions being inproximity to the lateral edges of the molten film.
 2. An apparatus ofclaim 1 wherein each of the lateral portions of the film in proximity tothe uninsulated electrode comprises less than 5 percent of the totalwidth of the film.
 3. An apparatus of claim 1 wherein each of thelateral portions of the film in proximity to the uninsulated electrodecomprises less than 3 percent of the total width of the film.